1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for producing the same. In particular, the present invention relates to a liquid crystal display device having liquid crystal molecules which are axially symmetrically aligned in liquid crystal regions separated by a polymer wall, and a method for producing the same.
2. Description of the Related Art
Conventionally, TN (twisted nematic)-type liquid crystal devices or STN (super twisted nematic)-type liquid crystal devices have been used as a display device employing electrooptic effects. Technologies to enlarge a viewing angle have actively been developed.
As one of the technologies for enlarging the viewing angle which have been developed, Japanese Laid-Open Publication Nos. 6-301015 and 7-120728 disclose a liquid crystal display device having liquid crystal molecules which are axially symmetrically aligned in liquid crystal regions separated by a polymer wall. Such a device is commonly referred to as an ASM (axially symmetrically aligned microcell) mode liquid crystal display device. The liquid crystal regions substantially surrounded by the polymer wall are typically formed pixel by pixel. In an ASM mode liquid crystal display device, liquid crystal molecules are axially symmetrically aligned, and thus observers experience less variations in the contrast, irrespective of a viewing direction in which the observers view the display. In other words, such a device has a wide viewing angle characteristic.
An ASM mode liquid crystal display device disclosed in the above-mentioned publications is fabricated by polymerization-induced phase separation of a mixture containing a polymerizable material and a liquid crystal material.
A method for producing a conventional ASM mode liquid crystal display device will be described with reference to FIGS. 10A through 10I. First, a glass base plate 908 (shown in FIG. 10A) is provided with a color filter and electrodes formed on one side (upper surface) thereof. For simplicity, the color filter and the electrodes formed on the upper surface of the glass base plate 908 are not shown. A process of forming the color filter will be described later.
Then, as shown in FIG. 10B, a polymer wall 917 for axially symmetrically aligning liquid crystal molecules is formed, for example, in lattice on the surface of the glass base plate 908, where the electrodes and the color filter are formed. The polymer wall 917 is formed in lattice by spin-coating the glass base plate 908 with a photosensitive resin material, and then performing exposure and development using a photomask having a predetermined pattern. The photosensitive resin material may be either a negative type or a positive type. Alternatively, the polymer wall can be formed by employing a resin material with no photosensitivity, although a separate step of forming a resist layer must be added.
As shown in FIG. 10C, column-like projections 920 are discretely patterned on a portion of an upper surface of the polymer wall 917 thus formed. The column-like projections 920 are formed in a discrete manner by patterning a photosensitive resin material on a portion of an upper surface of the polymer wall 917, and by performing proximity exposure and development.
As shown in FIG. 10D, the surface of the glass base plate 908 is coated with a vertical alignment material 921 such as polyimide or the like so as to cover the polymer wall 917 and the column-like projections 920. Thus, a substrate is formed. Likewise, as shown in FIGS. 10E and 10P, a counter glass base plate 902 is also coated with the vertical alignment material 921 so as to cover an electrode (not shown) formed thereon, thereby forming a counter substrate.
As shown in FIG. 10G, the two resultant substrates are attached together in such a way that the surfaces having electrodes are facing inward. In this manner, a liquid crystal cell is formed. A gap between the two substrates (i.e., a thickness of a liquid crystal layer described later; referred to as a xe2x80x9ccell gapxe2x80x9d is defined by the sum of the heights of the polymer wall 917 and the column-like projections 920.
As shown in FIG. 10H, a liquid crystal material is injected into a gap in the liquid crystal cell thus obtained by a vacuum injection method or the like, thereby forming a liquid crystal layer 916. The liquid crystal layer 916 is divided into a plurality of liquid crystal regions 915 (only one is shown in FIG. 10I) by the polymer wall 917. As shown in FIG. 10I, liquid crystal molecules in the liquid crystal region 915 are controlled to be axially symmetrically aligned with respect to an axis 918 (shown by the dotted line) which is perpendicular to both the glass base plates 908 and 902. The liquid crystal molecules are thus controlled by, for example, applying a voltage between a pair of electrodes respectively provided on the glass base plates 908 and 902 and facing each other.
A cross section of a color filter is shown in FIG. 11. A black matrix (BM) 510 and a color resin layer 512 including a red color (R) pattern, a green color (G) pattern, and a blue color (B) pattern are formed on a glass base plate 508. The red, green, and blue color patterns each correspond to a pixel. The black matrix 510 blocks light passing through a gap between the color patterns. An overcoat (OC) layer 514 formed of an acrylic resin, an epoxy resin or the like is provided on the black matrix 510 and the color resin layer 512 to a thickness of about 0.5 xcexcm to about 2.0 xcexcm so as to improve the smoothness and the like of the surface of the color filter. On top of the overcoat layer 514, a transparent signal electrode 516 formed of an indium tin oxide (ITO) layer is further provided. The black matrix 510 is generally made of a metal chromium layer having a thickness of about 100 nm to about 150 nm. As the color resin layer 512, a resin material colored by a dye or pigment is used. Generally, the thickness of the color resin layer 512 is about 1 xcexcm to about 3 xcexcm.
The color resin layer 512 is formed by patterning a photosensitive color resin layer formed on the glass base plate 508 by photolithography. For example, by forming, exposing and developing red, green, and blue photosensitive color resin layers (i.e., each step is repeated three times in total), the color resin layer 512 including red, green, and blue patterns can be fabricated. The photosensitive color resin layers can be formed by applying a liquid photosensitive color resin material (diluted with a solvent) on a base plate by a spin-coating method or the like, or by transferring a photosensitive color resin material in the form of a dry film onto a plate. By fabricating the above-described ASM mode liquid crystal display device with such a color filter, a color liquid display device having a wide viewing angle characteristic can be obtained.
However, when the above-described ASM mode liquid crystal display device and the method for producing such a device are applied to a large liquid crystal display device having high resolution display characteristics, the following problems described with reference to FIGS. 12A through 12D will arise.
As shown enlarged in FIGS. 12A through 12D, the polymer wall 917 and the column-like projections 920 are formed in such a way that their side surfaces are inclined (i.e., tapered) with respect to the base plate 908. Such an inclination is inevitable in proximity exposure usually employed in photolithography performed for a large base plate, by which a photomask and a base plate are exposed in the state of being proximate to each other. The reason is that a proximity gap (i.e., a gap between large base plate and a correspondingly large photomask) cannot be extremely reduced. In the case where the proximity gap is extremely reduced, the base plate and the mask are occasionally in contact with each other due to a warp or a flexion of the base plate and the mask. In order to prevent any damage caused by such a contact between the mask and the base plate, the proximity gap must have a certain size (about 100 xcexcm). Accordingly, a relatively wide spread of light passing through the mask, or the like, results in a formation of the polymer wall 917 and the column-like projections 920 in such a tapering manner. The relatively wide spread of the light also causes the polymer wall 917 and the column-like projections 920 to be larger than the pattern of the photomask. As can be understood from such a phenomenon, it is difficult to form the polymer wall 917 and the column-like projections 920 in a microscopic pattern.
In the structure shown in FIGS. 12A and 12B, since the height h of the polymer wall 917 is relatively high and the width thereof is wide with respect to the cell gap d, the column-like projections 920 can easily be provided on the polymer wall 917. However, when the height h of the polymer wall 917 is constructed as high as shown, the polymer wall 917 may act as a resistance against an injection of the liquid crystal material into the gap of the liquid crystal cell. Such a phenomenon increases time required for injecting the liquid crystal material and thus lowers the throughput. Especially when subjecting a thick layer to photolithography, the resultant size of the polymer wall 917 and the column-like projections 920 occasionally become larger than the pattern size of the photomask by up to a several ten percent. Accordingly, the width of the polymer wall 917 is increased, so as to reduce an opening width of the liquid crystal region 915 by that amount of increase, thereby reducing a numerical aperture of the liquid crystal display device. These problems are particularly conspicuous when the pattern of the polymer wall and the like are formed to be microscopic in order to produce a high resolution liquid crystal display device.
Therefore, as shown in FIGS. 12C and 12D, there is a case where a polymer wall 917xe2x80x2 having a smaller height hxe2x80x2 and a smaller width is preferred. In such a structure, as compared to the structure shown in FIGS. 12A and 12B, the numerical aperture of the display device can be increased, thereby improving a brightness of the display and reducing the time required for the injection of the liquid crystal material. Yet, as can be seen in FIGS. 12C and 12D, the relative height of column-like projections 920xe2x80x2 is increased, while the width of the polymer wall 917xe2x80x2 is decreased. As a result of the narrowed width of the polymer wall 917xe2x80x2, a bottom portion of the column-like projections is formed to be extended beyond the width of the polymer wall 917xe2x80x2. In particular, since the resin layer forming the column-like projections 920xe2x80x2 is thick, there is a possibility that the bottom portion of the column-like projections 920xe2x80x2 is formed of greater width than the mask pattern by a several ten percent. A part of the column-like projections 920xe2x80x2 is formed also within a liquid crystal region 915xe2x80x2, which reduces the numerical aperture of the display device. Furthermore, when a part of the column-like projections 920xe2x80x2 is formed within the liquid crystal region 915xe2x80x2, the axially symmetrical alignment of the liquid crystal molecules is disturbed, and as a result, a leakage of light, for example, is generated in a black display state, thereby inducing a flickering in images being displayed.
According to one aspect of the invention, a liquid crystal display device includes a first substrate; a second substrate; and a liquid crystal layer interposed between the first substrate and the second substrate. The first substrate includes a polymer wall extending in a first direction and in a second direction intersecting the first direction. The liquid crystal layer includes a plurality of liquid crystal regions separated by the polymer wall, and liquid crystal molecules in the plurality of liquid crystal regions are axially symmetrically aligned with respect to an axis perpendicular to a substrate surface. The first substrate further includes a column-like projection, and the column-like projection and the polymer wall define a gap between the first substrate and the second substrate. The column-like projection is provided in an intersection region where a portion of the polymer wall extending in the first direction and a portion of the polymer wall extending in the second direction intersect each other.
In one embodiment of the invention, at least one part of the column-like projection extends into a portion of the polymer wall beyond the intersection region.
In another embodiment, the column-like projection has a quadrangular area facing the substrate surface, and the at least one part of the column-like projection includes at least one corner among four corners of the quadrangular area.
In still another embodiment, four sides of the quadrangular area are at a 45 degree angle with respect to the first direction.
In yet another embodiment, the column-like projection is provided on the polymer wall and has a side surface inclined with respect to the substrate surface, and the polymer wall has a height lower than that of the column-like projection.
According to another aspect of the invention, a method for producing a liquid crystal display device including a first substrate having a base plate, a second substrate, and a liquid crystal layer interposed between the first substrate and the second substrate, the liquid crystal layer having a plurality of liquid crystal regions separated by a polymer wall is provided. The method includes the steps of forming a polymer layer on the base plate; patterning the polymer layer, thereby forming the polymer wall extending in a first direction and in a second direction, the second direction intersecting the first direction; forming a resin layer on the base plate to cover the polymer wall; and patterning the resin layer, thereby forming a column-like projection in an intersection region where a portion of the polymer wall extending in the first direction and a portion of the polymer wall extending in the second direction intersect each other.
In one embodiment of the invention, the resin layer is formed of a photosensitive resin, and the step of forming the column-like projection includes the step of patterning the resin layer by photolithography.
In another embodiment of the invention, the step of patterning the resin layer Includes the step of exposing the resin layer through a mask having a quadrangular opening, wherein the resin layer is exposed to light generated by a first, second, third and fourth light sources arranged in such a way that images thereof are located on diagonal lines and in the vicinity of corners of the quadrangular opening, thereby forming the column-like projection having a bottom surface corresponding to the quadrangular opening.
In yet another embodiment, the resin layer is formed of a transparent resin.
According to the present invention, the column-like projections are provided on the intersection of the polymer wall, so that the column-like projections can be formed to have a wider bottom surface than the width of the lattice pattern of the polymer wall without causing an adverse effect on the liquid crystal molecules in the liquid crystal display regions. Thus, even in the case where the size of the pattern of the polymer wall is reduced by refinement of the liquid crystal display device, the column-like projections can be placed on the polymer wall. Furthermore, column-like projections of a large liquid crystal display device are generally formed by utilizing a proximity exposure, and thus, side surfaces of the column-like projections are inclined with respect to the substrate surface. According to the present invention, even in the case where the large liquid crystal display device is structured in such a way that the polymer wall is low and the column-like projections are relatively high, the column-like projections can be formed in positions which give no adverse effect on the liquid crystal display regions.
According to the present invention, the column-like projection is provided in such a way that a bottom surface thereof is at a 45 degree angle with respect to the lattice pattern of the polymer wall. As a result, even in the case where the column-like projection is formed to be of offset from the polymer wall, into the liquid crystal regions, none of the corners of the column-like projection is in the liquid crystal regions. Thus, a disturbance in the axially symmetrical alignment of the liquid crystal molecules is prevented. Accordingly, even in the case where the polymer wall and the column-like projections are offset from each other, the axially symmetrical alignment of the liquid crystal molecules is not disturbed, as long as such an offset is kept within half of the width of the polymer wall. This enables an increase in an alignment margin for a production process.
Moreover, when the column-like projections are produced to be greater than are originally designed, the axially symmetrical alignment of the liquid crystal molecules is not disturbed for the same reason described above, and thus, a process margin of the production can be increased.
A patterning to form the column-like projections is implemented by exposing a resin layer via a mask having a quadrangular opening. During the exposure process, it is preferable to arrange images of first, second, thirds and fourth light sources in such a way that those images are placed along diagonal lines and in the vicinity of the apices of the quadrangular opening of the mask. The resin layer is exposed to light generated by the first, second, third, and fourth light sources thus arranged to form the column-like projections having a bottom surface corresponding to the quadrangular opening. As a result, the column-like projections with a bottom surface extending to in the directions of the diagonal lines of the quadrangular opening can be constructed. Therefore, the column-like projections do not extend beyond the intersection regions of the polymer wall, so as to prevent the column-like projections from adversely affecting the axially symmetrical alignment of the liquid crystal molecules.
When a transparent photosensitive resin is used to form the column-like projections, an alignment mark can be recognized even when the entire substrate is coated with the transparent resin. Thus, an accurate position alignment can be achieved without complicating the production process.
Thus, the invention described herein makes possible the advantages of providing a liquid crystal display device with a characteristic having a wide viewing angle and providing high precision and bright images, and a method for producing such a liquid crystal display device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.